Java Collections

Java Collections CS 15-121 Ananda Gunawardena

What is a collection? • A collection (sometimes called a container) is simply an object that groups multiple elements into a single unit. • Collections are used to store, retrieve and manipulate data, and to transmit data from one method to another. • Examples: • Arrays, hashtables, vector, set, map, tree

ArrayList Collection • ArrayList is a java collection that allows management of a dynamic list collection • Eg: • ArrayList A = new ArrayList(); • A.add(new Integer(10)); • System.out.println(A.get(0)); • If (A.contains(new Integer(10))) { ….} • A.remove(0);

ArrayList API (some methods) See others at http://java.sun.com/j2se/1.4.2/docs/api/java/util/ArrayList.html

Exercise • Given an ArrayList A of objects write a method removeDups that removes all duplicate elements from the list

Introduction • The Collections Framework • A unified architecture for representing and manipulating collections, allowing them to be manipulated independently of the details of their representation. • Reduces programming effort while increasing performance. • Allows for interoperability among unrelated APIs, reduces effort in designing and learning new APIs, and fosters software reuse.

Collections Framework • Interfaces: abstract data types representing collections. Interfaces allow collections to be manipulated independently of the details of their representation. In object-oriented languages like Java, these interfaces generally form a hierarchy. • Implementations: concrete implementations of the collection interfaces. In essence, these are reusable data structures. • Algorithms: methods that perform useful computations, like searching and sorting, on objects that implement collection interfaces. These algorithms are said to be polymorphic because the same method can be used on many different implementations of the appropriate collections interface. In essence, algorithms are reusable functionality.

Interfaces • A Java interface contains specifications for a known set of functions. • These functions are to be implemented by classes • Eg: public class foo implements List { } • Interfaces from java collections framework

Core Collection Interfaces

Examples of using Collections

Creating your own generic type Java does not allow above Here is a hack

Benefits of Collections • Reduces programming effort • Increases program speed and quality • Allows interoperability among unrelated APIs • Reduces the effort to learn and use new APIs: • Reduces effort to design new APIs • Supports software reuse

The Set interface • Set is a collection with NO order • Similar to the Mathematical Abstraction of Set public interface Set { // Basic Operations int size(); boolean isEmpty(); boolean contains(Object element); boolean add(Object element); // Optional boolean remove(Object element); Iterator iterator(); boolean removeAll(Collection c); boolean retainAll(Collection c); void clear(); // Bulk Operations boolean containsAll(Collection c); boolean addAll(Collection c); //Array Operations Object[] toArray(); Object[] toArray(Object a[]); } • Example: Collections noDups = new HashSet(c );

Examples of using Set import java.util.*; public class FindDups { public static void main(String args[]) { Set s = new HashSet(); for (int i=0; i<args.length; i++) if (!s.add(args[i])) System.out.println("Duplicate detected: "+args[i]); System.out.println(s.size()+" distinct words detected: "+s); } } • What is the output of : java FindDups I am sam sam I am

More Set Operations • s1.containsAll(s2): Returns true if s2 is a subset of s1 • s1.addAll(s2): Transforms s1 into the union of s1 and s2 • s1.retainAll(s2): Transforms s1 into the intersection of s1 and s2 • s1.removeAll(s2): Transforms s1 into the (asymmetric) set difference of s1 and s2 • Exercise: Write a program that will read a line of words and list those ones that occur once and those that occur more than once. • Example: java FindDups i am Sam Sam I am • Unique words: [i, I ] • Duplicate words: [Sam, am] • More info at: • http://java.sun.com/docs/books/tutorial/collections/interfaces/set.html

The List Interface • List is an ordered Collection(sometimes called a sequence). • Lists may contain duplicate elements. In addition to the operations inherited from Collection, the List interface includes operations for: • Index based Access: manipulate elements based on their numerical position in the list. • Search: search for a specified object in the list and return its index position. • List Iteration: extend Iterator semantics to take advantage of the list's sequential nature. • Range-view: perform arbitrary range operations on the list.

The List Interface public interface List extends Collection { // index based Access Object get(int index); Object set(int index, Object element); // Optional void add(int index, Object element); Object remove(int index); abstract boolean addAll(int index, Collection c); // Search int indexOf(Object o); int lastIndexOf(Object o); ListIterator listIterator(); ListIterator listIterator(int index); // Range-view List subList(int from, int to); }

List Implementation • Two important List implementations • ArrayList • LinkedList

Iterators • Iterators allow collection to be accessed using a pre-defined “pointer” public interface ListIterator extends Iterator { booleanhasNext(); Object next(); booleanhasPrevious(); Object previous(); intnextIndex(); intpreviousIndex(); void remove(); // Optional void set(Object o); void add(Object o); }

Iterators Examples • What is the purpose of the following code? for (ListIterator i=L.listIterator(l.size()); i.hasPrevious(); ) { Foo f = (Foo) i.previous(); ... } • Steps through a list backwards

Map Interface • Maps keys to values • Each Key can map to a unique value public interface Map { // Basic Operations Object put(Object key, Object value); Object get(Object key); Object remove(Object key); boolean containsKey(Object key); boolean containsValue(Object value); int size(); boolean isEmpty(); // Bulk Operations void putAll(Map t); void clear(); // Collection Views public Set keySet(); public Collection values(); public Set entrySet(); // Interface for entrySet elements public interface Entry { Object getKey(); Object getValue(); Object setValue(Object value); } }

HashTable • HashTable is an implementation of the Map interface • An example of generating a frequency table import java.util.*; public class Freq { private static final Integer ONE = new Integer(1); public static void main(String args[]) { Map m = new HashMap(); // Initialize frequency table from command line for (int i=0; i<args.length; i++) { Integer freq = (Integer) m.get(args[i]); m.put(args[i], (freq==null ? ONE : new Integer(freq.intValue() + 1))); } System.out.println(m.size()+" distinct words detected:"); System.out.println(m); } }

Algorithms • The Java provides a reusable algorithms that can operate on a List. The following algorithms are provided. • Sorting • Shuffling • Routine Data Manipulation • Searching • Composition • Finding Extreme Values

Sorting • A List can be sorted according to its natural ordering • Java uses an optimized merge sort • Fast: runs in n log(n) time • Runs faster on nearly sorted lists. • Stable: It doesn't reorder equal elements.

Example import java.util.*; public class Sort { public static void main(String[] args) { List list = new ArrayList(); list.add(“guna”); list.add(“bob”); //etc… Collections.sort(list); System.out.println(list); }

Shuffling • Shuffle algorithm does the opposite of sort • It puts elements in the list in a random order so that each element is equally likely • Applications • Shuffle a card deck • Example: • Collections.shuffle(list);

Useful Algorithms • Reverse • reverses the order of the elements in a List. • Fill • overwrites every element in a List with the specified value. This operation is useful for reinitializing a List. • Copy • takes two arguments, a destination List and a source List, and copies the elements of the source into the destination, overwriting its contents. The destination List must be at least as long as the source. If it is longer, the remaining elements in the destination List are unaffected. • Swap • swaps the elements at the specified positions in a List. • addAll • adds all the specified elements to a Collection. The elements to be added may be specified individually or as an array. Source: java.sun.com

Search • The binary search algorithm provides a way to search for a key. • Example • int pos = Collections.binarySearch(list, key); • if (pos < 0) l.add(-pos-1);

Composition • Frequency • counts the number of times the specified element occurs in the specified collection • Disjoint • determines whether two Collections are disjoint; that is, whether they contain no elements in common

Java Collections